1. Field of the Invention
The present invention relates to drying devices, and more particularly to a drying device adapted for improved and faster drying of a user's hands and hair.
2. Description of the Related Art
Conventional hand dryers dry an individual's wet hands in one of two ways, evaporative drying or "blow off" drying. Conventional evaporative hand dryers include a blower for generating an air stream through a ducting system to an exit nozzle which directs the air stream onto the hands of the user. The air stream is heated by a heating device to evaporate the moisture on the user's hands. The hand dryers generally include a push button or other means to actuate the blower and heater for a predetermined time period (i.e., 30 seconds).
The drying time for these conventional evaporative hand dryers is relatively slow, taking thirty (30) seconds or more to dry a user's hands. The typical commercial hand dryer is rated at 20 amperes at 110 volts which means that it delivers at a rating of 2.2 KW. That is enough energy in 30 seconds to evaporate about 30 grams of water. But the average amount of water on wet hands is three grams or less. Thus, conventional dryers are only about 10 percent energy efficient. The energy loss is a result of the following operating factors: heating up the internal dryer components; not maximizing and optimizing air flow temperature, direction and velocity; not compensating locally for evaporative cooling; and not addressing the problem of a boundary layer of water molecules which inhibits evaporation at the skin surface of the hands. Attempts to improve energy efficiency in the prior art include providing an enclosure for the hands, recirculating air, predrying the air and use of infrared (IR) radiation as the primary heating means.
As mentioned above, a boundary layer of water molecules retard evaporation. When wet systems (i.e., hands) are impinged by an air stream, a layer of very low velocity, very high humidity air forms at the surface which creates a barrier to the transfer of heat and also to the removal of evaporating water. This layer of air which forms at the surface as a result of the difference velocity between surface and air is called a boundary layer or stagnation layer. Since the air in this stagnation layer does not move, it will form a zone in which water molecules leaving the water film surface just below it will accumulate, thus rendering it saturated. Since this layer it not readily swept away, the higher water vapor concentration in the boundary layer results in water molecules in the boundary layer condensing back to the wet film surface, due to random motion, thus reducing the net rate of evaporation.
Conventional hand dryers that use "blow off" or "air knife" technology do not use evaporation (although a small amount occurs) but instead, provide an intensive blast of high velocity air which when suitably deployed, blows or skives droplets of water off the user's hands. The difficulty associated with this technique is that it requires expensive pressure blowers or compressors and nozzles having critically designed apertures. When drying is achieved by blowing away these water droplets, water will accumulate in the vicinity of the machine--walls, floor, the user--unless provisions are made to collect and dispose of the water.
It has been found that after using a conventional "blow off" hand dryer, the hands feel cold and slightly moist, possibly as a result of some presence of the boundary layer of water-saturated air on the hands as well as some cooling due to heat of evaporation from that small portion of the moisture that has evaporated off. Other disadvantages are that the hands must be inserted into a narrow enclosure which provides a collection trough to collect the droplets of blown off water. The collection trough includes a drain hole which feeds a floor level container that requires periodic draining of water by maintenance staff. Further, problems are then introduced in keeping the accumulated water from accumulating mold and bacteria with time. These "blow off" hand dryers are, therefore, inherently complex, noisy, large and expensive while at the same time requiring dedicated ongoing maintenance.